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Historical frost averages plus extension guidance, not a forecast. Check the forecast and soil temperature before planting.
Start onion seeds indoors 10–12 weeks before your last spring frost. Plant sets or transplants outside 2–4 weeks before that frost date, once the soil can be worked. Onions are hardy and handle light frost well.
- Frost tolerance
- Hardy: frost-tolerant
- Soil temperature
- Sprouts at 35°F+, best 50–95°F
- Days to harvest
- 90–120 days from seed; less from sets
The rule we use
- Start seeds indoors: 10–12 weeks before the last spring frost
- Plant sets or transplants: 2–4 weeks before the last spring frost
Pick the right day-length type
Onions start forming bulbs when days reach a certain length, so the type has to match your latitude. Short-day onions bulb with 10–12 hours of daylight and suit the South, roughly below the latitude of North Carolina. Long-day onions need 14–16 hours and suit the North. Intermediate, or day-neutral, types work across the middle of the country.
Plant a long-day onion in Texas and it may never bulb. Plant a short-day onion in Minnesota and it bulbs early and small.
Seeds, sets or transplants
Sets are small dry bulbs. They're the easiest to plant and the most forgiving, but large sets often bolt to flower. Pick sets about the size of a dime.
Transplants, sold in bundles or grown from seed, give the best bulbs. Sow seed indoors about ¼ inch deep, keep it around 70°F until it sprouts, and trim the tops to about 4 inches tall so seedlings stay sturdy. Harden off for a week before planting out.
Planting and growing
Set sets or transplants about 1 inch deep and 4 inches apart in rows 12–18 inches apart. Onions have shallow roots and don't compete well with weeds, so keep the bed weeded and evenly moist.
Harvest when about half the tops have fallen over. Pull them, let them dry in a shady, airy spot for 2–3 weeks, then cut the tops and store them somewhere cool and dry.
Seeds and supplies for onions
- Onion seeds and sets at True Leaf Market
- Park Seed for seeds and young plants.
- Seedling heat mats keep seed trays warm for fast, even sprouting.
- LED grow lights stop seedlings stretching toward a window.
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Questions about planting onions
When should I plant onion sets?
About 2–4 weeks before your last spring frost, as soon as the ground can be worked. Enter your ZIP above for dates.
Why are my onions not forming bulbs?
Most often the variety doesn't match your day length. Long-day onions grown in the South, for example, may never bulb.
Can onions take a frost?
Yes. Onions are hardy and young plants handle frosts in the upper 20s °F. A long cold spell after planting can make them bolt, though.
Sources
- Vegetable Planting Calendar for Central Maryland (2026), University of Maryland Extension
- Vegetables for Spring and Fall Planting; Vegetables for Summer Planting, University of Maryland Extension Master Gardeners
- Suggested Vegetable Planting Dates for Utah, Utah State University Extension
- Vegetable Planting Guide (CMG GardenNotes #720), Colorado State University Extension
- Soil temperature conditions for vegetable seed germination, Oregon State University Extension
- Extension Gardener Handbook, Ch. 16: Vegetable Gardening, NC State Extension
- Frost dates: NOAA NCEI U.S. Climate Normals 1991–2020 (doi:10.25921/4ek7-fk11)